Is there any feat or mythic path ability I can take that would allow me to get an attack of opportunity when they melee attack someone else while in my reach?
The main way I can think of doing this is the Broken Wing Gambit teamwork feat. It does come with some restrictions however. It requires the ally being targeted to have first hit with a melee attack, gives opponents a bonus to their attack and damage roll, and as a teamwork feat requires both allies to have it to work. The second issue isn't that large if you're expecting the AoO to put the enemy out of action before their attack resolves, and the third can be mitigated if you have an ability like the Cavalier's Tactician that lets you share teamwork feats (I'm not sure abilities that let you count allies as having your teamwork feats would work for this though: they would actually need to have the feat to take the free action to 'open up').
As a side note, the feat does have an upgrade, Wounded Paw Gambit, that lets the setup work with some ranged attacks as well.
A feat that (mostly) does what you are looking for is called Archon Justice, the capstone of the Archon Style tree:
Archon Style:
While using this style, as a move action, you can protect adjacent allies from a single opponent that you currently threaten. This grants adjacent allies a +2 dodge bonus to AC against that opponent’s next melee attack (as long as that attack comes before the beginning of your next turn), but causes you to take a –2 penalty to AC against that opponent until your next turn. The dodge bonus against the designated opponent’s attacks persists even if your allies move away from you.
Archon Diversion:
The penalty to AC for using Archon Style to grant nearby allies a bonus to AC against a single opponent decreases to –1. Once per round while using Archon Style, when you have at least one hand free or when wielding a shield, you can divert one melee weapon attack targeting an adjacent ally to yourself. After resolving the attack, the ally you protected can make an attack of opportunity against the diverted opponent. You expend no action to divert the attack, but you must be aware of it and must not be flat-footed. You must choose to divert the attack after the opponent has declared the melee attack, but before the attack roll is rolled.
Archon Justice:
You no longer take a penalty to AC for using Archon Style to grant nearby allies a bonus to AC, and you can activate this effect as a swift action or a move action. Whenever you take damage from an attack you diverted to yourself using Archon Diversion, each ally threatening the attacking creature can make an attack of opportunity against that creature.
This combo has prerequisites of BAB +8, Combat Expertise and Combat Reflexes, plus Archon Style and Archon Diversion, but no class requirements. It also won't work unless you're adjacent to your ally.
You are your own ally, so not only do you get an AoO for this, so does your buddy being attacked, and any other ally in melee range.
You could also accomplish this for just yourself with just the second feat in the tree, Archon Diversion, by playing a class that gets a parry/riposte mechanic such as a swashbuckler or duelist, then successfully parry-riposting the diverted attack.
The only ability I could find (that wasn't one of those mentioned by others) that comes close to that is the Mythic Guardian's Sudden Block, which allows the Guardian to hinder an attack against himself or an immediately adjacent ally (adds your tier to the AC, the attacker rolls twice and takes the worst roll) and then either you or your ally can counter with an AoO.
Much like Archon Style, it only works when the ally is next to your PC, though.
