You haven't provided enough information. But based on what you've provided, my tentative answer is, upgrading to Mountain Lion will render Windows unbootable.
Mac OS depends on GPT. Windows on Apple hardware depends on MBR. MBR only supports four partitions. In a "normal" situation with Boot Camp there are already four partitions: EFI System, Macintosh HD, Recovery HD, Windows. There is no more room for a fifth "FAT32" partition to fit into the MBR. Upon seeing this fifth partition, Apple's tools appear to wipe out the hybrid MBR, and replace it with a PMBR (single entry MBR, making the disk GPT only), and that renders Windows unbootable.
If you want to do yourself and your parents a favor, put Windows 7 in a VM instead of using Boot Camp. It's a huge PITA, especially if anything goes wrong. It literally invites data loss in my opinion.
----------
To provide more information for a less tentative answer, please post the results for the following Terminal command. Please highlight the pasted text, and click the # button in the forum toolbar to properly format it.
Code:
sudo gpt -r -vv show disk0