When using the comparative with environmentally friendly would it be correct to say environmentally friendlier, or more environmentally friendly?
Although they both might be technically correct, I'd say it's harder to parse the latter one, that being:
more environmentally friendly
Here it might seem like more relates to environmentally, thus trying to create the phrase more environmentally, which in the former example doesn't happen:
Here it's pretty clear the comparative is friendlier, exactly as you want it to. I'd go for this one.
Either form is perfectly acceptable and neither should be considered incorrect, as friendly is part of a set of adjectives that can form a comparative with either the y -> ier transformation or more + adj.
However this is quite an unusual case because friendly is an adjective that looks like an adverb. It used to be an adverb but hasn't been used as such for a very long time. Most -ly adverbs only use the more + adv. comparative form. Perhaps this could be one reason that more environmentally friendly is preferred.
The form could also depend on context. For example,
We need more environmentally friendly buses.
is potentially ambiguous. Is this a call for buses to be friendlier to the environment (probably) or for more buses?