it depends on how close is the firendship.

Closeness is defined on a shoe string. Overly concern on the "friend" usually will results in

1. comparison with your current partner
2. feelings will somewhat creep into your own life
3. imagination running wild with What IF scenario
4. entanglement and hard to disengage if goes too deep

Hence as normal friends is OK but as close friend is a NONO and dangerous game.