Is it considered hiding assets because he gave her money for the down payment on the house when his name is not tied to the house in anyway?
Absolutely. Unless he claims it was payment for sex, and then you can get her for prostitution.
This is why it is important to value everything as of November. Whatever he has done with his income and (his portion of) assets in the meanwhile is immaterial. The farther you get from that date, the harder it is to calculate.
The flip side of the argument is that if he wants to value everything at a later date, you can argue that his time and talents going into OW's house were marital assets, along with his "gift" of money for the downpayment.
