Will Mountain Lion destroy 3rd data partition?

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.

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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

 

OK so not to be a dick, but what part of Please highlight the pasted text, and click the # button in the forum toolbar to properly format it. wasn't understandable?

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The GPT already contains 4 partitions. I can't tell you for sure if Mountain Lion will successfully wedge in a new partition for Recovery HD (Lion tended to fail to do this frequently). If it's successful the Windows partition will not be bootable. You'll need to use gdisk to create a new hybrid MBR, where you will add the last two partitions to the MBR (you'll need to check the GPT again after the upgrade to see what the partition numbers are).

In effect this stuffs the GPT, EFI System, and Macintosh HD into MBR partition 1 as type 0xEE (GPT protected partition) and thus those partitions will be ignored by Windows. It will only see the last two which are "Windows basic data" partitions. Mac OS X will continue to see all of the partitions since it uses GPT and ignores the MBR.

Again, I advise using VM for Windows.

 

Yeah, I was able to figure it out, so the same answers as earlier apply. The MBR can't hold 5 partitions, and currently Apple's tools fail to create a hybrid MBR when the GPT contains 5+ partitions, and without a hybrid MBR Windows will not boot on Apple hardware.

 

Or go down the rabbit hole of using gdisk to create a new hybrid MBR after you've done the Mountain Lion upgrade. Instead of adding every GPT partition to a corresponding MBR partition (which you can't do since you'll have 5 GPT partitions and MBR can only hold 4); you can add just the last two GPT partitions (FAT32 and Windows 7) to the MBR. The earlier GPT partitions will all be defined in the MBRs first partition (one MBR partition for three GPT partitions) marked as the GPT protected partition.

I advise VM. I advise VM. I advise VM.

With VM, your disk management is VASTLY simpler, less fragile, disk space consumption is far less yet still growable without having to worry about resizing later, and it's easier to backup and restore down the road should they outgrow this drive.

 

My setup was very similar and I upgraded with no problem.

Originally, 3 usable partitions:

1. OS X, Snow Leopard
2. Fat32, Shared data partition
3. Win 7

The upgrade to Lion preserved all 3, did not attempt to create a recovery partition.

The upgrade to Mountain Lion, same thing. All 3 partitions preserved intact, just the OS X partition upgraded.

You can separately create the recovery partition on an external HD.

Of course, backup all partitions first, in case your experience is different.