[aidenn0]

Helium is an inert gas, so is extremely unlikely to be part of any chemical reaction (much less a bio-chemical reaction like breathing).

[edit]

Methane or ammonia appear to be much more likely alternative, and no I wouldn't like Earth's atmosphere to be replaced with it.

While I'm being a nitpicker Terra-form means roughly "to make like Earth" so it specifically refers to making a planet more like Earth.

[grecy]

OK sure, Helium was a bad choice. Substitute your own.

> While I'm being a nitpicker Terra-form means roughly "to make like Earth"

Substitute whatever word the aliens have that means "to make like home planet" then.