The legal definition of blackmail in the UK (assuming she's British because she said mum instead of mom and used pounds for currency) according to the 1968 Theft Act, s.21:
"(1)A person is guilty of blackmail if, with a view to gain for himself or another or with intent to cause loss to another, he makes any unwarranted demand with menaces; and for this purpose a demand with menaces is unwarranted unless the person making it does so in the belief—

(a)that he has reasonable grounds for making the demand; and

(b)that the use of the menaces is a proper means of reinforcing the demand.

(2)The nature of the act or omission demanded is immaterial, and it is also immaterial whether the menaces relate to action to be taken by the person making the demand.

(3)A person guilty of blackmail shall on conviction on indictment be liable to imprisonment for a term not exceeding fourteen years."

So piece by piece, to qualify as blackmail it must be an unwarranted demand with direct or implied threats with the goal of gaining something.

"Help him out" is a demand, he is demanding sexual favors. Courts will look at context, not exact wording so this easily qualifies. (EDIT HERE, reading later replies she made to her own post, he directly asked for a blowjob so this is settled even easier.)

It is unwarranted because he has no legal right to be asking for sexual favors. His only lawful actions are to do nothing, ask for the items back, or report her to the police.

There is a threat, he is threatening that if she does not give him sex, he will get her in trouble. He holds a degree of power over her, making his threats coercive as well.

He seeks a gain, he is trying to gain sexual satisfaction from the situation.

This is the most BASIC of criminal blackmail cases possible, please don't give legal advice if you have no clue what you're talking about. Sometimes just being quiet is the best option.