You probably already know that the maxed out utilization is what's killing their scores. If there is any benefit for them, it would be extremely minimal, if anything at all. People with scores of 750+ and acceptable UTI might find it difficult to get a personal loan for the amount you're talking about, let alone one with a decent APR.
Hopefully they've stopped using the cards and are looking at other avenues to earn some extra money to throw at this debt until it's gone.
I think their best shot would indeed be through a credit union.
I am going to opine that it won't help but for a different reason. There was a time when an AU from anyone would boost credit scores. People used to "sell" trade lines i.e. the use of one as an AU to boost scores. The bureaus got wise to that and shut down that avenue of score improvement. From what I have read/heard unless the AU is a spouse/parent/immediate family member or living in the same house/address such as long term couples living together then the bureau won't really increase the score that much and instead interpret as an artificial attempt to boost the scores. Hypothetically if the score was boosed a bit by the lowered over all utilization I think creditors are still going to be gun shy in this economic climate of giving a personal loan to someone with over 95% utilization on all the cards they personally opened. That factor alone will weigh heavily in any decision making.
This person needs the snowball method and to sock drawer if not cut up those cards until they get their spending under control. I agree with the other response that said enabling them isn't going to fix this situation.
