If it was any other time, then perhaps Ash may not have allowed it to gain something like that, especially when on a job with others.
Usually she would not have taken the risk, but she needed its power to grow, and there was nothing better to aid that than an Evil Spirit like Black Avalanche.
There was nobody around at that time, so she did not have to worry about witnesses seeing it absorb that Life Energy after claiming parts of it for itself.
She could sense the others approaching enough to know how far away they were, and she knew enough of her surroundings to know how much they could see from those positions.
That had allowed her to have a little more security in making her decision to allow it to feast on what it could claim in such a short time before moving to another location to hide away.
