Can Catholic priests disclose the fact that someone has seen him for confession?
The short answer is possibly, but it is certainly not looked on in a well conceived mannered way.
In the absence of a clear Catholic source from Canon Law stating that it is or is not allowed, it is to be considered as being allowed.
AthanasiusOfAlex is right on specs here and is correct.
While revealing that someone has been to confession does not violate the seal, common sense says that it is best not to provide information that could inadvertently reveal a person’s sins—and revealing whether a person has been to confession or not could potentially do this in one way or another.
I will give a little anecdote here.
One day, many years ago, when I was in a lineup to go to confession there was a person known to many as not practicing his faith, somewhat in front of me. He went in and eventually come out of the confessional.
After Mass, a rather imprudent person asked the priest in question if so and so had been to confession to him. His response, I will always remember: Possibly, but that is not your concern!
The question was never brought up again!
For the rest, I will let Canon Law answer this question:
Can. 983 §1. The sacramental seal is inviolable; therefore it is absolutely forbidden for a confessor to betray in any way a penitent in words or in any manner and for any reason.
§2. The interpreter, if there is one, and all others who in any way have knowledge of sins from confession are also obliged to observe secrecy.
Can. 984 §1. A confessor is prohibited completely from using knowledge acquired from confession to the detriment of the penitent even when any danger of revelation is excluded.
§2. A person who has been placed in authority cannot use in any manner for external governance the knowledge about sins which he has received in confession at any time.
Can. 985 The director of novices and his associate and the rector of a seminary or other institute of education are not to hear the sacramental confessions of their students residing in the same house unless the students freely request it in particular cases.
Can. 986 §1. All to whom the care of souls has been entrusted in virtue of some function are obliged to make provision so that the confessions of the faithful entrusted to them are heard when they reasonably seek to be heard and that they have the opportunity to approach individual confession on days and at times established for their convenience.
§2. In urgent necessity, any confessor is obliged to hear the confessions of the Christian faithful, and in danger of death, any priest is so obliged.
Admitting that someone has been to confession is not the same thing as revealing what sins may or may not have been confessed. Logic is clear on this!
If ever answered this question would be of interest here!
Proof of sacramental absolution required for marriage in France during the French Revolution?