Also have to factor in that the clients she sees are punters - ie people predisposed to higher risk behaviour. On that count I arbitrarily assume the risk is doubled. So now we have a 1/21 risk of a potential transmission chain.
I think just doubling the risk is a vast under-estimate.
Your lass as you say has a much higher risk of being exposed to the virus than the average person even if she only saw clients who were averagely at risk, because she is having close, pro-longed physical contact with multiple strangers.
But the chances are that the clients are not averagely at risk, they may also be having close pro-longed physical contact with several other wgs, who in turn are a much higher risk than average and so it goes on.
I would assume that the chance an active prossie seeing 4 or 5 punters a day over a few weeks will be exposed to the virus is pretty much 100%. It's just a case of whether you see her before she gets it, or after she has recovered and hopefully can't pass it on.
But lets not forget that even if she hasn't got it, if her previous clients that day, or in the previous few days, were carrying it, and touched anything in her premises that you then touch, you could get infected that way.
It really isn't very sensible to be seeing prossies right now, and some would say it's down-right irresponsible, but several people I like are doing so so I guess one can't preach to loudly.