I agree with Mr. Wanerman, as far as his answer goes. I would add two things. First, there being no warrant the burden of showing consent and voluntariness is on the police. This makes the argument to suppress easier. However, if your friend did not live there and had no key to the locked garage where the evidence was found, he may not have standing to challenge the search. This question could hinge on how long he had been visiting his mother's home, whether he was sleeping there at night, and whether he had personal items in the house that show that he was a temporary resident. You need to consult a lawyer on this one, it is a complicated issue.

This is a difficult question. Consent given by an incompetent person or given under duress is not consent. So, it certainly sounds like the police did not have valid consent to search. However, a search conducted based on consent that the police honestly believed to be valid (which they will certainly claim they did) may be valid. The police may also claim other exceptions to the warrant requirement such as hot pursuit (unlikely if your friend was in a locked garage) or exigent circumstances (the police believed your friend or someone else was going to destroy evidence if they didn't go in and get it). You need to consult an attorney in your area and provide him all the details for proper advice.