Can invocation of nehek heal a lone character?
Yes.

According to Forming Units (8th Edition Rulebook, pg 5), "A unit usually consists of several models that have banded together, but a single, powerful model such as a lone character, a chariot or a Dragon, a war machine and its crew, and so on, are also considered to be a unit."

Everything is a unit, even a lone character.

Though bear in mind the rules for Invocation of Nehek. "Models with the Vampiric, Ethereal or Large Target special rules can never regain more than 1 Wound per successful casting." (Vampire Counts 8th Ed, pg 60). So a lone Vampire for example will only gain a single wound.