If, on the other hand, they have their PHD on the back-burner and are not really putting enough into it, while living the high life on a studentship that a well to do student could be getting, then I think its a problem. Basically, everything hinges on how well they are doing. If they are doing poorly, I assume that questions would be raised as regards to why. If it was because they weren't commiting enough time, and they had a job that contravined the Terms & conds, then I imagine there'd be trouble.
In summary then: theres only a problem if it can be noticed by the effect on their work.