It's wrong to contact your ex's partner.
Valerie is dating Will and is considering moving in with him.
Valerie wants to know more about how Will is as a roommate.
Valerie messages Will's college roommate and friend to ask how he is as a rommate.
Valerie messages Will's ex-girlfriend to ask how he is as a roommate.
Valerie will get a fair perspective because Will's roommate has no grudge against Will.
Valerie will get a skewed perspective because Will's ex might bear a grudge.