What's making you post the question?
I'm assuming the initial incident wasn't a foul?
Proximity can help sell decisions like that, and it probably gives you the chance to say a bit more. From a distance all you can do is shout 'No! Nothing in that, keep playing!', a bit close and you can go say a bit more to just Red 2 'He couldn't help it, your bloke knocked him over!' or some such. Although some referees would argue against that - start a conversation like that and you're inviting argument. Invite argument, and it erodes your authority.
You're hoping to either gain the Red 2's trust, or build a very quick rapport. Failing that, if you can see he has murder in his eyes, 'nothing silly!'...and he'll duly ignore it
If he thinks he was fouled and is going to react, there's a fair chance nothing you did could have stopped it. But on some occasions it may enough. If it isn't, you've made your initial opinion clear and loud, and hopefully that will make it easier to sell any card you may have to give to Red 2.
Anyway, Red 2 has gone in for a heavy tackle and caught his opponent with his studs with some force. That has to be a yellow card at minimum. Given it's going to be clear there was some retaliation, you want to make it clear you won't tolerate it at all.
The fact that he's clearly retaliating makes it more sinister though, and depending on the force used in the tackle, a red card would certainly be possible.
Free kick for red 2 for a careless action, intent isn't in the laws
Heavy studs up tackle to the foot sounds like a yellow card at minimum but hard to judge force when you aren't there.
Be careful with your terminology. A careless tackle is a FK only. If it's a yellow card, it's reckless.
