Transmission flush

My son has been told that his vehicle needs a transmisison flush. Having never heard of this procedure, I have tried to do some research and have become totally confused. Is this considered necessary maintenance for a high mileage vehicle. Would changing the transmission filter and fluid suffice? I've read that it is best not to have this done unless you have the money available for a new transmission and engine? I realize that my vehicle maintenance knowledge is "old school" and I'm not familiar with many of the new procedures, (also, being that my daughter-in-law is who has been told this needs to be done and we all know the horror stories of how women are taken advantage of by some mechanics) but my research has failed to clairfy wheter this is necessary or not. Thanks in advance for your advise ?