Sure, you're within your rights to tell her that. You're also within your rights to kneecap her with a baseball bat. But don't expect her to like either one.
Divorce is war; battle lines are drawn, sides are taken. Your friend is counting on you and her other pals to stand with her, not her enemy. If you tell her you want to stay friends with her ex, she'll feel betrayed, even if you remain friends with her, too. People who are divorcing often have a "for me or against me" attitude, especially early on. Is it fair? No, but emotions run high and out of whack during a divorce.
Now, if this is one of those oh-so-rare amicable divorces, she might not care if you continue to pal around with her ex. But I wouldn't count on that. Like I said -- it's rare.
Unfortunately, odds are you'll have to make a choice between your friend or her soon-to-be ex. Yes, that sucks, but it is what it is. I won't try to tell you who to choose, although there is something to be said for the fact that you were friends with her first. On the other hand, if you get along better with him, he might feel shocked and betrayed if you choose her.
It's a pickle, and you gotta eat it, sorry to say. Whatever choice you make, someone will lose and be hurt. That's divorce for ya -- every day's a new flavor of suck.
Good luck. You're gonna need it.
The man speaketh the truth. Things might change once the initial fury has passed, but in when the shit first hits the fan you pretty much have to pick your camp and stick with it.
Choosing sides is never easy. When a friend tried to "seduce" (I use the word loosely, just like the friend was *snap*) my boyfriend, I decided to cut ties with her. Since a lot of us were all connected it meant some people choose to cut ties and others didn't. I didn't try to influence anyone's decision but I'm not gonna lie, it did hurt to see some of my friends who still are in contact with her. I couldn't help but think about her, and all the inappropriate things she did, when I saw them, it made getting that person out of my mind a little harder. But I've respected their decision and have still stayed close. I realized that I liked her for a lot of reasons and those reasons don't have to go out the window for my other friends.
If you decide to stay in contact with the ex husband, wait a while and let your girlfriend know. She deserves to be told from you. Hopefully at that point she's emotionally strong enough to understand and remain friends with you. After all, at one point she loved him enough to marry him.