Hi collegegirl,
The earlier MBPs, like yours, have a separate Graphics Processing Unit that adds to the power draw when compared with the current "base model" 15" 2.53 model that comes with the 60W power supply. The newer computers also have a reduced power load in other areas, too.
I've run my own older SantaRosa 15" on 60W power adaptors on various occasions (more by accident than deliberate decision - we have several of both varieties in the house) and nothing untoward has happened, but they are likely to heat up a little more than an 85W when used on an "85W" computer, and your battery may also be "called on" to supply extra power on occasion, even when the charger is connected.
http://support.apple.com/kb/HT2346 has some advice, and lists the adaptors appropriate for different MBPs.
So, in short, though the solution probably seems appealing to you both, it is not really a good idea, I'm afraid.
Cheers
Rod