Depends on your organisations policies and prodecures. If it states that work at height is to be controlled by a PTW then it must be issued and followed to meet your own requirements. If on the other hand it is not a requirement and they procedure is repetitive then the relevant RA/MS should be sufficient.

And if you are setting your policy on this - you need to ask yourself if there is any benefit in permits for this type of work. Does it give any additional control or protection? It doesn't sound like it in this case.
There is a disadvantage in having too many permits (especially repetitive ones) of devaluing the whole permit system.

Entirely your choice of course. Where you have glazing reached via intermediate levels (e.g. flat roof) involving edge protection and/or fragile surfaces (skylights) then you may want to operate a PTW which gives assurance that the guys who turn up on the day are able to demonstrate (in all respects) a safe sytem of work?

There is another element to this, window cleaners do not work on their own sites and as such may have to comply with the site/ building owners SMS. For his own work on his own site its up to him if he uses a PTW, but when he is on other sites he may have to compy with his clients requirements.
clear as mud :-)

I usually decide (with others) that a permit isn't required if the work is in the main 'routine'.

Window-cleaning is routine work for a window cleaner so, presumably, risk assessment already done for most situations he encounters, and adequate controls in place re securing ladder, 3-point contact.

I'd say permit to work, only if some out-of-the-ordinary cleaning process is involved. e.g. different equipment, maybe a high traffic area (vehicles or people) requiring an exclusion zone, difficult environment making securing ladder difficult needing someone permanently in attendance etc.

And since he is working on someone else's premises a PtW would likely be appropriate whenever he uses a platform or tower scaffold.