With those levels, templates, and gear, I would hesitate to add any aid.
I'd love to hear how this turns out, however.
The Oinodaemon actually kinda looks like a wimp. All he really has going for him is his SR, AC, and the DC 46 Transfixing Gaze; he also lacks a lot of immunities that most extreme boss monsters tend to have so it is possible for him to get spell sucker punched and essentially one shot, but yeah SR 48 does help with that. Beyond that he hits like a b+#%$ damage wise even if using Power Attack and all his spell DCs are laughably low for a high level party.
You know your party; how hard really is AC 60 for them? Are your casters optimized to reasonably beat SR 48? I'd figure out there chances of succeeding on any given roll versus your dude. If both of those are fairly reasonable for your party to be able to handle then I would definitely give your big bad some help. Hell, you may want to anyway as he's not all that strong offensively anyway. Maybe spend all that loot he has and gear him up if necessary.