I do not know what halacha says but if there are such obvious violations of Shabbos going on, I think I would stay away from that shule.
In my personal opinion, someone who openly violates any mitzvoh
(like saying Loshon Hara, when they clearly know what they are saying is assur, and they make excuses for it, or if they wear untznius clothes like the Halachly forbidden ‘slit’, at the bottom of some skirts and dresses and have been living with frumkeit long enough that they clearly should know better, or were clearly told and refuse to change) is not “Frum”.