Yes, the DA needs proof of the violation of Probation and they must prove their case even with a VOP. I suspect its something other than the ex-wife, but only time will tell that little fact.
I suggest he hire an attorney to handle the matter for him either way because, regardless of the fact that its late in his probationary period, he could still go to jail for some additional time if they truly violate him.
This answer is not intended to form an attorney/client relationship and any answers do not constitute direct legal advice and should not be followed unless and until you have spoken with an attorney of your choice.
If there is evidence that he trespassed (which simply means entered a property after notice was given that he could not enter or notice was given to leave), then his probation can be revoked. (You do not say what the probation is for but if it is deferred, which the majority of probations are, then he can be sentenced up to the maximum term regardless of how far along he is in the probation.) The burden of proving a probation violation is significantly lesser than proving the case itself. (In fact, they could dismiss the trespass charged and still use the charge to revoke his probation.) He will NOT be getting off his probation as long as there is a motion to revoke probation pending (which may not happen - the probation could just be allowed to expire - it will be up to the judge of the probation charge court.)
If the criminal trespass is set for trial, the ex-wife, if she is the witness, must appear in court. There is no trial by affidavit, letter, or even deposition in Texas criminal courts.
You can help him by hiring him a lawyer to represent him on the new charge & the potential MRP (motion to revoke probation.) You can help him get to court every time on time.
If he is on bond and a motion to revoke probation is filed, there will be another arrest or at least another bond to be posted. Be aware.
Although I have answered the question to try to help you, you should consult with a lawyer in your area in person on the matter. In addition, my answer does not establish an attorney-client relationship between us.