The way I understand your post is that you've bought Reforged on an account that already had classic Warcraft 3 registered and you were hoping that you'd get an extra set of classic cd-keys that you could later give to your friend, so you could play together.
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Well, if that's the case, I will have to disappoint you - Reforged pre-purchase doesn't come with a set of extra cd-keys for classic Warcraft 3. What happens is that if you buy Reforged on an account that didn't previously have classic Warcraft 3 registered, the account will get automatically updated to also include the older version of the game, allowing you to play it instantly. However, if your account already had your old cd-keys added to it, you will not receive a free version of classic Warcraft 3 with your pre-purchase.
In such case, I can see 2 options:
1) You can try submiting a ticket to Blizzard Support, asking them to award a classic Warcraft 3 licence to some other account, because the one you've bought Reforged on was not eligible to receive a free copy of classic Warcraft 3, but... I highly doubt that Blizzard will agree.
2) What I'd consider doing in your position is trying to get a refund on your pre-purchase - since it's a pre-order, it shouldn't be that hard to get. Once you get a refund, you can go ahead and register another Battle.net account and then pre-purchase Reforged on it, so that it gets a free copy of the classic game. Then you can just share your account data with your friend and have him play on your account for the time being. Once Reforged comes out, you can try asking Blizzard to merge both of your accounts so that ultimately you're not stuck with 2 separate accounts - I don't think that Blizzard will have any issues with that.
However, be mindful that this technique will require you to share your account with someone, which:
a) Is against the EULA and while I don't think that something like this could get you banned, I'd advise against going around and telling people (especially Blizzard's support agents you will contact to merge your accounts) about this.
b) Always puts you at risk. If you share your account data with someone, there's always a possiblility that this person will steal your account, so be careful.
c) There's always a slight risk that Blizzard will frown at such tactics and/or might refuse to merge both of your accounts.