Marvin,
Even though I too have a tendency to take my laptop wherever I go, I can't see hauling it along on vacation to PP!!! Maybe your wife needs a vacation away from the keyboard...
Well, if she insists on bringing it along, your wife probably will not have problems getting through Customs with the laptop, esp. if carried in a shoulder case. I lived and worked four years in the DR, came and went numerous times with my laptop. Only asked about it once, and I said it was a computer I use in my work, that I'm a writer (easier than explaining writer/consultant/researcher). The officer nodded and that was that. If you wish to be on the cautious side, take a copy of her sales slip for the computer with you, or go to the Customs office at the airport you're departing from in the US (presume Canada can do this too) and ask for the declaration form that shows they stamp that attests that the item in question is a personal use item which was in your possession when you left the US. Probably won't need it, though.
As for other things to know, I would STRONGLY suggest full surge protection for both the electrical connection and modem connection. Most telephone lines in DR get surges; I fried two modems and a fax while living in the DR, and once that was when I supposedly had line surge protection.
If she really plans to use the laptop much, I would also urge using a Lithium Ion battery, or if that is not possible, take a second battery along. Blackouts are unpredictable, and even hotels supposedly equipped with generators can be affected. Low voltage can also affect some computers. She'll need the batteries to tide her over through such periods.
Last but not least, protect the laptop. Don't leave it lying around you room. It may not be there when you get back to your room. A good laptop can sell for quite a bit in the DR and would prove a tempting target for a thief.
Hope this helps.
Regards,
Keith