Acol Strong Twos -bidding problem

Here is a bidding sequence from a recent competition.

The opening bid was 2 hearts a strong Acol two showing 8 playing tricks.

The reply was 2 no-trumps which is negative showing less than 7 points.

The opener's rebid was 3 clubs and the responder then bid 3 spades.

At this point opener passed. She argued that opener had limited his hand by bidding 3 no-trumps and she could therefore tell that no game was possible (having only a singleton spade).

Responder argued that a new suit at the three level is always forcing,

The actual hands were:

H A,K,Q,J, x
C A,K,Q, x, x
D Q, x, x


H xx
C xxx
D xxx
S KQxxxx

No game was made and the hand was really a misfit best played in a 5-3 club fit making 10 tricks.

What is your opinion? Is 3 spades forcing?