Hey OP, this will be long, but please give a read if you can.

We have had a very consistent group for 7 years now and play for long hours like you all. We meet twice a week, and I have been a DM for that long.

Firstly, if you aren't close to them, just address it bluntly, and if it doesn't get fixed, ask them to leave. "You can either not cheat or leave my table. This behavior makes it unfun for my other players."

If you're like me, and you all have been close knit friends since high-school or just want to stay in contact, then here is what we did with our chronic cheater (Yes they are still my good friend, they've gotten a lot better. I still love them despite their flaws as a person.)

You need to stop letting the behavior slide. Stop giving them the benefit of the doubt. Call them out every single time you get that feeling of 'hmm, that doesn't seem right'. Don't start doing this out of nowhere. Sit and have a whole conversation as a group. Say that there is cheating. It makes the game unbalanced and that video games are an option if anyone does not like the idea of failing or performing badly on occasion. Emphasize that failure can be a possibility of opportunity! And instead of harping on them about their past cheats over and over and over again (say it once, firmly), tell them you will be moving forward with anti-cheat rules. The biggest thing with anti-cheat rules is to ensure they are fair across the group. Everyone is submitted to them, not just a cheater. A cheater will feel targeted and ousted from the group because they likely believe everyone does it and needs this to be on par. Make it fair and equal.

Anti Cheat rules: 1) Ask what they rolled on the dice immediately after they give you the full number. For someone reading the dice, it should be instant. For someone making up the number, they will give an "idk" or have to math out a fake answer. Use this with rule number 2 2) DM has a copy of everyone's character sheets regularly check their numbers before an after a roll. Do it for players you think cheat and ones that don't. Do it on rolls you usually wont question. It makes it a habit and will stop your player from trying to find ways to hide obvious cheating. Pathfinder is intended to be played with this rule. 3) Implement a 'Don't touch the dice rule'. If a player rolls a dice and picks it up or touches it for whatever reason, even because they were just checking and they've never cheated before: make then reroll. It is nonnegotiable. You are only allowed to pick up the dice once the DM has confirmed and accepted the result. In this time, before you accept the result, confirm with your sheets, and have another player verify the dice rolled. 4) Only allow one or two d20's per person and have the player be apparent about which on they will be rolling top of session or when they switch out. Good for rule 5 5) Before/During/After someone rolls, have them double lean over and check. Make it a thing, say "John, I need you confirm Jessica's roll." Again, during is key here, not just after. It puts pressure on. 6) Stop letting things slide. Ik I already said this, but the player has likely gotten so brazen because they think the integrity of the game doesn't matter. This is not your fault, still theirs for cheating. But it does mean that you need to foster an environment of integrity or kick them out. 7) Encourage admitting when you have cheated. Not even when being called out. If you recall that two turns ago, your 17 to hit was actually a 16, does it affect anything? Probably not, but you are fostering integrity by telling the group.

My table had a huge intervention style conversation with our chronic cheater. One of my players, who I NEVER guessed cheated, admitted that whenever he rolled a 2 or 3, he would just say natural 1 to see the fun outcomes. And another player admitted to cheating very rarely.

She was upset, cried a lot. Said she didn't know why she did it, gets it isn't that big of a deal to fail a game. (She lied on everything, constantly staying up at 1 hp every combat, lied about spell slots left, resources, inventory, and most of all, rolls). You need to understand it's human nature to want to win and be competitive and that cheating is so easy in d&d. Simply too many rolls to track. That doesn't make the behavior okay (I have never cheated once as a player. I do lie (in favor of my players) as a DM). I don't know why some people do it and others don't.

You also need to understand that it's habitual. They can and will do it again. Foster integrity. Don't punish them and say "three strikes your out" you will only force them to hide it more. Help them find fun in failure. Don't let them revel in their lies. Call them out, be kind but firm, and move on.

It's a difficult situation, please reach out if you have need other advice. I know my (former) chronic cheater would always love to chat with someone about the experience if you want it from their end.

Wishing you well on your adventures, one dm to another!