If you think a friend's new love interest isn't good enough for them, you have every right to voice your opinion to them: but should you? This is dangerous territory, and it might be best to let this love affair just take its own course. By butting into other people's love lives, you run the risk of alienating them. After all, if they know you don't like their sweetie, then they're not going to be spending lots of time with you. Bite your tongue and observe things for a while.