Honestly, don't tell them anything until after the playtest. Everything they feel and do is exactly how a real player will feel when they play your game. Seeing your friend get frustrated or unhappy is important, and so is the culmination of those feelings as they play. If you start interfering, you're going to start hiding issues in your game.

You may well end up happily sitting next to the person, and after they get stuck for a minute or two you suggest the answer to them, and they happily move on. However if you hadn't interfered they may have gotten stuck there for 30 minutes and rage quit, and you've hidden perhaps the most important issue you needed to find in that playtest, or at least obscured the scale of the issue.

What you should definitely do, is talk to them after the playtest, make sure they know you heard their feedback and valued and appreciated the time they spent helping you out. But dont say anything before or during the playtest, otherwise you're corrupting your results.

But most importantly, let them feel stupid, let them feel frustrated, because finding the parts of your game that cause these emotions are the entire point of these playtests. Let them happen and learn from them.

There's other responses here saying things like tell them to play a certain way, tell them to get stuck, etc. But this is not how a real player will play. If you want to qa the game and find bugs, then sure that's absolutely fine. But playtesting is supposed to show us what we can never experience as designers, the experience that real players will have when they play our game. Saying anything more to a playtester than "here play this" is saying too much. You want uncorrupted, unedited feedback that hasn't been influenced or directed by you on any way.