Should I add GFCI outlets in my bathrooms if those circuits already have a GFCI remote on them?
Your friend is out of his league here

A GFCI outlet has two sets of terminals on it -- LINE terminals for the power IN as well as LOAD terminals that connect to the GFCI's protected hot and neutral in addition to that protected hot and neutral being provided to the GFCI's face receptacles.  Hardwired applications can use what's called a "faceless" GFCI which just has the LOAD terminals instead of having both LOAD terminals and receptacles on the front; there are also GFCI circuit breakers that integrate ground-fault, short-circuit, and overcurrent protection into a standard circuit breaker package.

However, series GFCIs won't break anything

However, it is possible and permissible to connect two ground-fault trips in series -- nothing is harmed by that configuration, and it's more common than most people think.  The only problem you cause by that is confusion as it is indeterminate (i.e. not defined) which GFCI will trip in response to a ground fault.