My sons ex partner keeps posting things on Facebook saying my son was violent & warning women to beware of him as well as other things.

We have all the things she has posted. Also she keeps sending emails of which she states "she is helping others very shortly he will get a big shock which will cost him a few grand in compensation + the opponents legal costs". My son is in the process of a divorce & this indicates his ex partner is in contact with his wife for what ever reason. She also stated in the email (a job he has applied for) her friend who apparantly works there "knows what your about so good luck if your considering taking the job". I would like to know if its possible to send his ex partner a solicitors letter to say all this defamation of his character must stop or we will take action? Thank you.