Given that you've increased the atmospheric pressure, the slight drop in O2 won't be a problem for breathing but your problem in this mix isn't the helium; it's the xenon.
Helium gets a bad rap because some people do stupid things with it; for fun. Yes, breathing helium in large quantities makes your vocal chords vibrate faster because it's less dense than normal atmosphere, so you sound like a chipmunk. The problem is that you need O2 in the air to survive. Breathing in massive amounts of pure helium suffocates you because you don't get any O2 by way of breathing it in. That said, it's not unsafe to breathe as part of the atmosphere and deep sea divers actually do exactly that; Heliox is a gas mixture of both O2 and He2 (instead of O2 and N2) because at pressure, the amount of nitrogen you breathe can make it toxic. Diving to even a relatively shallow depth of 30m using normal air brings on something called Nitrogen Narcosis and it affects your judgement et al like alcohol does.
Xenon, on the other hand, is also safe to breathe in its pure form but it has a singular disadvantage; it can be used to form many different compounds with O2 and many of those compounds are toxic to breathe and very exothermic, read as explosive.
In response to comments, it should be noted that these compounds are VERY hard to form and involve a significant amount of fluorine, more than would be allowed within trace gases. The paragraphs that refer to these reactions have been deleted, however the remainder of the answer is still valid.
The Xenon does however have a narcotic effect in its pure form, just like Nitrogen does in larger quantities than normal atmospheric pressure afford. Xenon is about 25x times more narcotic than nitrogen, so in the quantities listed above your colonists may not be able to function very well, if at all. It would be like living in a mist of anesthetic, and in point of fact xenon isn't used for anesthetic purposes in medicine now only because of its high cost.
Add to that you're dealing with gasses with very different densities. Xenon is heavier than normal air, helium is lighter. If the gasses start separating as a result (very likely) you could actually have a suffocating layer at different altitudes, and mine shafts in particular could be very dangerous as over time they'll fill with much more xenon than normal gasses.
Also, to have that much helium in the atmosphere in a stable form, you've probably got a LOT of exposed decaying uranium. So, while you have helium in abundance, you also have radiation.
All things considered, I'm not convinced this atmospheric model is stable, or safe. The helium isn't the real problem, but the way it's created might be. The xenon has the opposite problem insofar as it's not dangerous in a pure form, but what it becomes might be.
My recommendation? Have your colonists wear rad-safe spacesuits until they've done a more complete study of the atmosphere. Better safe than sorry.