Will an arm lift help for crepey loose skin on a lean person?

It depends on where the loose skin is occurring and how crepey the skin has become. In my experience, arm lift does help and whatever is left behind can be treated with a collagen builder like  Sculptra. It's similar to treating cellulite in the video below.

If there is excess skin, the arm lift will help.  Many patients come in with sun damaged crepey skin with which surgery may not be indicated.  If there is poor elasticity, treatments such as microneedling, radiofrequency or combinations may help to improve the collagen and elasticity of the skin.During the consultation, the plastic surgeon can best advise whether surgery or non-surgical modalities (or both) would be ideal.