Okay that is good, with "2.4" processor never sure if 4,1 or 5,1.
Means you do have good OEM 10.6.4x DVD you can use.
What I would do is some migrating and cloning with some spare extra or new good disk drives.
Not knowing if your friend geek installed a fresh ML on its own or reformatted hard drive or not - and did they "upgrade" to ML over Snow Leopard (probably).
ML dropped Rosetta, no more PowerPC code support.
ML is 64-bit.
Anything left over from SL and before can cause trouble.
And two things to help clean it up and get it up and running smoothly:
A. Clean install of ML and then use Setup Assistant
B. Clone you new good ML and keep it handy on the shelf
C. Clone your data to a new drive use CCC checksum
D. Old drives are now your backups. Common rotate your backup sets (current, copy of current, system frozen in time from earlier, like 10.8.2 before instal update to 10.8.3)
So when you say you have SL archived, dual boot is one way and then you can boot SL when you or a project needs SL for some reason, and use ML the rest of the time. You just may also want to install 10.6.5 and do all the upgrades when you can and be careful and keep it "clean" and see how it works.
Depends a lot on time of course but install and update of SL and ML is.... a Saturday morning?
Also depends on disk drives and budget.
I have my system on small SSD. I also have a disk drive for media files. And found that having graphic editing library on its own drive - all 3 drives operate independently and much quick and no delays.
SSD $100+ 120GB Samsung 840 is nice
WD 10K 500GB 180MB/sec $140
WD Black 2TB $165 or 1TB $90
WD 3TB RED (NAS and RAID) $120 - great fast reliable backup media - I also put a clone of the system (150GB) to have that handy at all times.
I think it is just the disk drive(s), Rosetta and any old extensions, background processes, plug-ins, and possible file corruption. A defective or "weak" disk sector (some sectors you have to reformat to map out, Lion and ML will test the partition tables better before accepting a drive partition).
* At a minimum you should be booting from ML RECOVERY volume and use Disk Utility to Repair twice: the raw hard drive (tests partitions and boot on system drive) and the normal repair on the user volume(s).
Follow up with a Safe Boot and Restart.
Knowing if you have any PPC programs that are not ML friendly and certified would help of course.