Christmas Munchkin Boxing Day - Wassail combo question

[Sublevel27]

We had 2 Level 9 players in combat for the win. One was an Elf Helper.

They had a bunch of Santa Monsters against them.

Then the main player played "Boxing Day" [from the Holidazed expansion] which allowed her to add a Santa Monster from her hand to fight on her side.

At one point, the Monsters were in the lead. Then the Elf Helper played the "Wassail" one-shot [also from Holidazed] which adds +7 to help a Santa Monster.

The Elf asked me (the GM) if he could play it to help their Boxing Day Santa Monster playing for the Munchkin side.

As I read the text, I said yes. The card says to help a Santa Monster, and since they came from the same expansion, I interpreted that they could use the +7 for the Munchkins benefit through the Santa Monster fighting FOR them.

They won the game with this combo. (All the other players were cleaned out of any further means to help the Monsters.)

Did I make a legal call?