Sugar-laden diets combined with both a mineral poor diet (ergo, no dental remineralization) and a constant subclinical dehydration (did you drink your gallon of water today? no? saliva is the vehicle for said remineralization, and is one of the first to be hit during early stages of dehyrdation; water is required to maintain the pH of your mouth) is bad your teeth.

The worst coffee can do is stain already poor enamel. Coffee is hardly acidic, and its a myth that its more destructive.

Now, if you drink your coffee with sugar? Thats bad, learn to drink it black.

https://www.ada.org/en/~/media/ADA/Public%20Programs/Files/J...

Anything on this list that is below a pH of 5 you should strongly avoid. pH is a log scale, so, pH of 5 is 10x worse than 6, 4 is 10x worse than 5 or 100x worse than 6; the hydroxyapatite in tooth enamel starts to dissolve at 5.5, and coffee is only 5.11, well within your mouth's ability to handle.

If he warned you because you suffer from bruxism, not because of tooth decay, for most people, just follow the normal rules for coffee that everyone else should follow: avoid caffeine 6 hours before bed.