I think I'd point out that she is being compared with peers from other schools, and some of her options at whichever senior school she ends up with may depend on what level she is at on entry. Being in lower sets at a comprehensive can be quite dull if she could be capable of more. [Of course a good school will ensure that she is stretched, but have a look at the comp site - it is likely that certain subjects will only be open to higher sets eg multiple MFL/triple science].
What set / stream she might find herself in at the comprehensive might depend on performance in SATs, or CATs / some other test once at the school, but it won't depend on how she does in the 11+. Equally, if it's likely that a pupil would naturally (i.e. without deliberately flunking whichever assessment) find themselves in low sets in a comprehensive school then it's unlikely that grammar school is a suitable environment for him / her.
Assuming that a place at the comprehensive is guaranteed anyway, I would acknowledge this, but push the 'passing the 11+ would just give more possibilities to consider' line (this is actually what we said to all of ours - apart from the guaranteed place at a comprehensive that we would have been happy with, which didn't actually apply, unfortunately).