Yeah, I don't think you can do that using counter attack as the heal wouldn't be targeting one of the enemies.
There are probably a few ways you could do this...here's the one that popped into my mind first. You'll need Yanfly's Skill Core.
1 - Give your troop a turn 0 event that gives your healer an invisible state. I'll call it CheckHeal. Give it an expiration at end of battle.
1a - If you use this mechanic repeatedly throughout your game, this state could just always be on as a passive state and you wouldn't need the troop event, or the expire at battle end, but I'm treating this as a one-off.
2 - Create a troop event with a span of Moment and a condition of Switch X ON (an unused switch number in your project).
3 - In the event, do Force Action from the appropriate enemy to cast silence on your healer, then turn switch X OFF
4 - In your healing spells, put the notetag:
Code:
<Custom Execution>
if (user.isStateAffected(stateId))
$gameSwitches.setValue(X, true);
</Custom Execution>
where stateId is the ID for CheckHeal, and X is the same switch ID as above.
5 - On the map where these enemies can be fought, make an event that has a blank first page; on page 2, give it a condition of Switch X is ON, trigger type autorun, and have it turn Switch X back OFF.
5a - If this mechanic is reused throughout your game and you're using a passive state the way I described above, instead of plopping an event like this on every map, you could just use a plugin to call a common event at the end of battle that turns the switch off.