Complete the 203B. The problem we all have with this scenario is that NY (and a few other states) demand that total wages be allocated to NY on the W2. (So if the client made $50k, $1k in NY, the state portion of the W2 will show $50k for NY, $49k for PA, making it look like he made $99k.) Only way out of this is to complete the B. On request, some employers will send a letter, based on expense reports, to break down the state income. Without that, you have to count the days.
The reason the states do this is that they want to tax you at your highest marginal tax bracket. In my example, if you only made $1k in NY you wouldn't owe any tax. If you made $50k, however, you'd be in the, say, 8% bracket. They want to make sure that $1k is taxed at 8%.