It would help for timing to know more specific than Taguig, where they're coming from. Rush hour around the terminal starts around 6:30 to 7 in my experience but I haven't been there in awhile. I would think 6 is plenty early enough, but if they don't mind waiting that extra 30 minutes of time at 5:30 a.m. would probably get them there for sure before the full rush hits. One of the more recent regulars to MNL might chime in.

If you really want to make it comfy get a hotel across from the terminal the night before and they can walk over at 8 a.m.