Forum:Forced to use an effect?

Hi, all. First time on the forum and only been playing Yugioh for about a week. I apologize to any forum moderator if this post is in the wrong thread or if this question is in the FAQ. I sincerely hope the moderators here are more friendly than the last place I was at.

Anyway, here's my situation.

I had Masked Hero Koga and Elemental Hero Neos in attack mode. My opponent had no monsters on the field and none she could summon in her hand. She drew a card that allowed her to take control of my Koga for one turn. She used it to attack my Neos. Both cards have an attack of 2500; however,Koga gets an additional 500 points for every monster your opponent controls. (I'll put the exact effects at the bottom of this post.) If she had used these effects, she would have taken 500 of my life points, but would have had to return Koga at the end of the round. It was much more advantageous of her not to use the affect. By not using the effect I didn't take the battle damage, but both Neos and Koga went into the graveyard. I let her do this since we're both new to the game. (Plus, it's just a game and I'm not going to get butt hurt about it.) However, I'm wondering what the ruling would be on this. Is that effect mandatory or can she choose to ignore it? I'm thinking it's mandatory but I welcome your opinions on this.

Here's the card's effects just in case you don't have it memorized.

Must be summoned with "Mask Change" and cannot be Special Summoned by any other ways. This card gains 500 ATK for each monster your opponent controls. Once per turn, during either player's turn. You can banish 1 "HERO" monster from your Graveyard then target 1 face-up monster on the field, that target loses ATK equal to the ATK of the monster banished to activate this effect. XrayPeregrine (talk • contribs) 20:51, May 2, 2015 (UTC)