The owner is renting the space to a Muslim group who are converting it to a Mosque. It will become a community space for the Downtown Muslim population and welcome others. Seems like it will serve a needed purpose in the area. The private owner of the church doesn't have to offer it up for the homeless, just like the majority of private developers aren't really doing anything significant to help the homeless crisis (beyond Devcore in Gatineau).
As for the washrooms, those can benefit the homeless population, especially if they remain open overnight. We have a seriously lack of public washrooms in Ottawa, especially washrooms accessible to the homeless population. If anything, we'd need the City to invest more in public washrooms.
We need to invest more in the homelessness crisis as well, and the City has made progress with increased OCH investments (with support from the Feds and Province, though Provincial policies has reduced private affordable housing) along with more transitional housing, but of course, we're nowhere near where we need to be.