Listen to these two. Do not steal/wear this womans panties.
Yes, it is possible that she might like it upon discovery and be happy about it, but odds are strong that the reaction will be other extreme because a girls panties are very personal to her and you should ALWAYS ask permission first before wearing ANY womans panties. If she likes idea, she let you know if it's ok.
Either way, if you do that anyways be prepared for a very strong reaction from her that may cause you to lose favor in this womans eye and I do not think you should risk losing her favor. You need her to like you. Not be mad at you.
What I think you should do is to buy your own panties. Preferably pink crotchless panties that you wear under your own undies. That way you get the thrill of them being discovered and give them added thrill when they discover them.
You can also leave them on during festivities.
This will be a guaranteed win for all parties involved. I feel very confident in saying that it will be a huge turn on for her, when finding you in them.
If nothing else, she will certainly want to know where you got them and if you bought them yourself. Which you must do if you do this. Stealing them from some innocent girl is wrong.
Go to store and tell sales lady its gift for your girlfriend. Buy a bra to match, too. It will make your purchase look like completely normal Vday present. Have them gift wrap it.
After all, valentines day is coming up. If there is EVER a time it is safe for any guy to buy girl lingerie safely, it's in weeks prior to valentines day. Anyone who sees will just think you being good boyfriend.
Also, as for just staying friends only, that my friend is most likely no longer an option. If you put your foot down and say no, but want to keep relationship same as before, it will either become weird feeling for you or them or both, or if everyone pretends "nothing happened" (lol) she will inevitably make little comments from time to time or otherwise tease you about it, as she now knows you like it, but are suppressing it.
After all, you never verbalized any resistance AND you came back and let it happen again.
Once is being curious.
Thrice becomes habit.
Fourth time signals addiction and you go back countless times from there and become her little bitch boi as long as she wants.
Fun times for you, my friend!
Had you stood up and said firm no right at the beginning, that would have been that and things stay normal. But now, all of you know that his cock been in you both ways and you cum from it. You really think things just go back to the way it was before? Every time you see each other now on, whether said or not, that's the first image that will be in mind about each other. Good luck with that.
You should just have fun with it and see where they want it to go. If they cross lines you don't want crossed in future, you can say so at that time. You tell them, "i am ok with this, but not so much with that. Can we do more of this and not that?"
Whatever "this" or "that" may be for you.