Hi,
Well I have to ask what size is your male and your girlfriends female? The male needs to be around at least 2 pd's smaller than the female, to help make sure the pups are small enough for her to deliver. It doesn't always work but there is alot better chance a smaller puppies and a easy birth for the mama.
If you have had all proper heath testing done and are going to breed then you need to know that different females are fertile at different times during the heat cycle, some females will conceive as early as 3-5 days into season. but most female are not ready until 9-12 days. Most breeders will breed on the 10-12-14th day of the heat cycle. Some breed on the 10th-11th and 13-th day. and some breed on the 9th-11th-13th day of the cycle. Usually when the female is ready she will start "flagging" lifting her butt up and down and wiggling it in the male dogs face and will let him mount and breed her. If the breeding is successful then the female should whelp 62-65 days after the tie! I hope some of this info was helpful to you.
And welcome to YT!! This is a great place to learn lots and receive lots of help and information about your yorkies.