Choosing to remain friends with your ex-boyfriend means that, in time, he will move on from your relationship and meet someone else. Congratulating your ex on his new relationship should be approached with caution to avoid sounding jealous or condescending. The relationship that you have with your your ex may also determine if congratulating him on his new girlfriend is appropriate, or if the subject is better left alone.

If you stay in regular contact with your ex-boyfriend, congratulating him on his girlfriend may not raise any eyebrows. On the other hand, if you rarely speak to your ex, congratulating him on his new girlfriend may come off as petty, creepy and give the indication that you are prying into his life or keeping a close eye on his social networking account.

The jury is still out on whether former lovers can be purely platonic friends. To avoid any awkward or ethically gray scenarios, congratulate your ex-boyfriend privately and without drawing much attention to yourself. Drawing attention to your congratulations may seem as if you are trying to seek attention from your ex and may send mix signals about your friendship with your ex and his new girlfriend. A simple text message or email may be the most appropriate way to send your best wishes, since the message can be delivered privately and discreetly.

What you say may be as important as how you decide to deliver your congratulations. If you have a close friendship with your ex, telling him that you're happy for him, that you think he and his new girlfriend look great together or that you really like his new girlfriend are simple ways to congratulate him. If you do not talk much, simply telling him that you are happy for him should suffice. Whatever you choose to say, say it genuinely and from the heart to offer an honest congratulations.

Don't compare yourself to your ex-boyfriend's new girlfriend and avoid making parallels between your former relationship and his new one. Attempts to compare yourself to your ex's new girlfriend may come off as jealous or petty. Attempting to rekindle your romance despite the fact that your ex has moved on may also be viewed as a social faux pas to your peer group and shared friends, in addition to being undermining toward your ex's new girlfriend.