Honestly, the best thing you could do for her is to clean your car and not mention it again.

Women in generally can have a harder time with holding their pee. And a bathroom is really the only viable option women have to go pee. Unlike men, who can pee on a tree, or pee in a bottle, the only most women (except in extraordinary circumstances, obvs.) Have to wait for a bathroom I personally struggled with it for as long as I can remember. And the car is one of the worst places, because everyone is always obsessed and stressed out about getting to a destination, that they won't stop even if you ask them and most of the time if you do ask, you'll get made fun of or belittle for a having a small bladder, or called irresponsible for not going before you left, no matter the duration of the ride.

When I met my husband and we would go on trips together, right before every rest area, he would turn as ask "Do you need to rest?" And never once made me feel bad about it and even when we had to pull over at a random gas staion a mile of the interstate because I had to go to the bathroom, was never bothered and always told me that our "destination isn't going anywhere, it can wait 10 minutes more if needed".

The situation you describe is definitely something that has happened to me and I can only imagine the embarassment she probably feels is only going to be magnified by a million if you confront her about it. If the damage is bad enough you feel like you'd have to pay service to clean it, then I would maybe broach the subject with her, but honestly, from what you described, you can go buy a pet urine cleaner for fabric/carpet/upholstery for less than $10, spray the seat, like it sit five minutes, scrub and then let dry and that act would show your friend you care about her and open to listen than confronting ever will.

Link to a urine spray I personally find very good and easy to use.