You might be looking for -

Ulterior

: existing beyond what is obvious or admitted; intentionally hidden.

(From Lexico)

Therefore, "ulterior motive" might fit your given context -

Ulterior motive

An alternative or extrinsic reason for doing something, especially when concealed or when differing from the stated or apparent reason.

(From Wiktionary)

Hidden agenda is an expression that may fit your context:

If you say that someone has a hidden agenda, you are criticizing them because you think they are secretly trying to achieve or cause a particular thing, while they appear to be doing something else.

(Collins)

The action of the “friend” in your example could be termed as 'ostensible', which means 'appearing to be one thing when it is really something else'.

Try this:

ruse
_{NOUN COUNTABLE}

A plan or trick for hiding your true intentions

_{Source: Macmillan Dictionary}

How about Dissemble:

To puts on a false appearance: conceal facts, intentions, or feelings under some pretense.

To disguise or conceal one's real nature, motives, or feelings behind a false appearance.

A dissembler is one who conceals his opinions, character, etc., under a false appearance; one who pretends that a thing which it is not.