Young friend died, no will, and girlfriend and mother fighting over property.

When a person dies intestate, that is, without a will, his property passes to his living heirs according to state laws that determine how property is distributed in these circumstances. The mother will be recognized as a living heir in the situation you describe.  The girlfriend will not. What were the rental arrangements?  Would it be possible in good faith for the girlfriend to take the position that her boyfriend was partially responsible for living expenses and to offset the value of the computer against those expenses?  Would it be possible for her in good faith to take the position that the computer was a gift?