Computer question

Here's a question for those who no doubt know a lot more about computers than I do, which would be about 99.9% of the computer using population. Oh, and I'm asking this on behalf of my mother and father-in-law, really.

They've got an older HP pc tower, probably about 4 years old and running what ever Windows operating system from that period. They have high speed internet through Mediacom, and the tower is hooked into their modem. She wants to get a MacBook pro, or ipad so she doesn't have to just sit glued to the desk where the computer is, but he still wants to use the HP, and she doesn't want him fiddling with the new Apple product. Part of the reason for wanting something new because he fiddles then she can't find what she's looking for.

So, here's the question. Is there a way they can set things up easily to still keep the Windows based HP tower, and also have wifi available for a new Mac? Is there such a thing as a spitter that would go in between the modem and connect to the pc and a wireless router to allow both to function?

Thanks guys.