I would just ask to take a girlfriend along, then nothing 'improper' will happen, even if the guy is not a 'real' artist.
I was doing a life class evening course years ago and the model failed to show. We were asked if anyone fancied modelling for that night. I said I would do it for the first hour (I went to nudist clubs in those days, so no big deal), if one of the girls did it for the second hour
.
Luckily, our youngest female student agreed, a girl still in her last year of school
, but when I stripped off, she realised that she had agreed to something (she thought we would be dressed!) without really thinking what I meant! She chickened out
, but my day was saved by our tutor (a nice middle-aged blonde ex-hippy type) who agreed to strip for the class. This (I should not have volunteered) was quite normal at her classes when a model cannot make it. She even came around amongst us, making critical remarks without a stitch on!