Your son needs to go through a formal eviction process to get her to leave then. However, he would be within his rights to change the locks back, but until she is actually evicted from the house (with her mother), he would be obligated to give her a key.
Unfortunately, while it is illegal for his wife to change the locks without his permission, it is not a crime (misdemeanor
or otherwise) so the police cannot really intervene and do anything other than ensure that your son has a key and to inform his wife that what she did was illegal.
If not, it could be considered an illegal lockout, and he too could get in trouble for doing that, although it would probably be considered a civil matter like the changing of the locks without your son's prior permission.
He can serve her with a three day notice to quit, which would mean that she would have three days from that date to leave the premises.
Afterwards, he would need to file a summons and complaint to have her formally evicted.
Here is some good and useful information on that point:
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