No, "não tem sal" means specifically "there's no salt", at least without context. We would put ela or ele in the sentence in order for that to mean "she/he doesn't have salt". PS.: entering the world of idiomatic expressions, "ela/ele não tem sal" or "ela/ele é sem sal" means that a person is not interesting or hot.
Hi RobHendelm! Well, if a food - or anything - are tasteless, so it has no salt, no sugar, and probably no spice at all. I guess this is what your wife's family wanted to say. If they are tasteless, they probably have no salt, but if they have no salt, it doesn't mean they are tasteless. For instance, an icecream has no salt, but it's far to be tasteless! Do you see?
For everyone who asking about how to translate is simply use infinite verb, like has no salt or have no salt means equal also in spanish portuguese and english you can avoid the person now the phrase não tem sal the verb tem is have, no there is or there are is different but you can perfomance as that well...