Can my boyfriend get sued for telling my "ex-friend" that if she hit me again, he would "strangle her?

Sounds like defense, i would generally refrain from attacking anyone that did not attack first, but if the girl attacks you he can use what ever force is needed to stop her attack. Some people don't really get that women can do a good amount of damage to a guy and if they are going to try and get physical they should expect a response.