When can I play the mate card?

[wun wun love]

Ok here's the situation:

My wife, a level 18 or so wizard, kicks down the door and gets something unimportant, like a cleric class card. She then decides she'll play the potted plant from her hand and make it a little tougher with the intelligent card. Then my son and I start piling on: he plays the ancient card on it and it's up to level 16 toughness.

Then I play King Tut (level 16) along with a wandering monster card. She considers for a moment and first plays a radioactive acid potion card (+5) before discarding the other 3 cards in her hand in order to use the wizards charm spell to charm the potted plant.

So, this leaves king Tut: she thinks she can easily beat him because she is level 18 (+5 for the potion) and he is level 16.

But then I play my mate card: now we have 2 level 16 monsters fighting for a total of 32 toughness.

She says this isn't fair, that I have to play all cards involving monsters upfront so she knows what she's fighting. To me the mate card is like any other enhancer and can be played at any time.

Thoughts? Who is right?!?!?