You can absolutely ruin your relationship doing this with the wrong person.
If this isn’t something you’re willing to let go or keep to yourself, and you are for sure going to pursue it, just know that the likelihood one or both of you will feel some negative/regrettable/painful feeling in the beginning is pretty high. Age and maturity plays a part in this as well.
Dont react too emotionally one way or another when you do it the first couple times. If the mood is right and you’re supportive, great, but the experience is very strange and overwhelming for most people who haven’t done it before. If it hurts or you regret it, talk about it AFTER it’s over and don’t start pointing fingers or get angry. Not only do you risk ruining it in the future, you run the risk of creating a rift of resentment in your relationship. If you fight a lot and/or there are jealousy issues, this might not be the right move. If the relationship is broken, swinging/cuckoldry
wont fix it.
That being said... assuming everything goes great, and you guys get used to it over time, remember to keep an open mind and communicate. Communicate. Communicate. You need to foster open, honest conversation and both parties should be comfortable sharing exactly how they feel.
That’s my long winded disclaimer before you take the plunge. I know you didn’t ask for it but I felt the need to say that before I answer your question.
Should you surprise her? No.
It’s a nice gesture and one day it’ll be something fun you can setup, but you need to stay a team through the process in the beginning as much as possible. Listen to her wants and needs until you “figure her out” and she lets you be the plug all on your own.