No, this isn't legal.  Your friend needs a lawyer's help.

If the check was made payable to your friend only, and her mother's name wasn't on the check, and if your friend is 18 or older, the mother must have forged her name.  It may be possible to get the money back, because of that, and let the bank go after the mother.

Even if your friend was under 18, that money was sent out for a specific purpose.  What her mother did was wrong, the only question will be exactly how it's described and dealt with.