Reported this, as my answer "He's going to chapel tomorrow" is also correct. As with mynd i'r ysgol and mynd i'r dre, the definite article is used in Welsh even when the English equivalent doesn't need one. If it wasn't, it would be impossible to convey the meaning "going to a chapel".
I've never heard of "going to chapel" in English. "Going to church", yes.
That said, I've heard of "going to mosque" on Duolingo discussion forums, which I've never heard myself, either, but then, I haven't been around a lot of English-speaking Moslems.
Broadly speaking, Anglicans/Catholics go to church but Nonconformists go to chapel. Welsh speakers became overwhelmingly nonconformist and so it is the Welsh norm to speak of going to chapel. (I've even heard it used when the building in question was a church).