Can you heal enemies in Frosthaven?
Not with a normal heal. Positive abilities target allies or yourself, and enemies are neither.
Heal is a positive ability, so it can only target allies or the figure performing it. An enemy is never a legal target for a heal unless the card says otherwise.
That also covers the odd case of healing objectives: an objective is not an ally, so a normal heal cannot touch it. Scenarios that want you to keep an objective alive give you a specific ability for it.
The mirror image is true for damage. Negative abilities target enemies, which is why a plain Attack can never be aimed at a party member.
“In general, negative abilities can only target enemies, and positive abilities can only target allies or the acting figure themselves.”
Rules change with errata and with digital patches. If your copy of the book disagrees, the book wins — the official Frosthaven FAQ is the next place to look.