Most reputable place to value large collection?

hello.

I know an older man that has a very large sports card collection. His health is failing and he mentioned to me that he doesn’t know what to do with his cards. Is there a reputable online place that the people in the sports card community trust to value something like this? Talking tens of thousands of cards. He used to own a sports memorabilia store 30+ years ago. He moved the binders and plastic cases to his basement and hasn’t touched them since. Sorry if this isn’t what this sub is for. Just trying to help the guy.