You're right; the Bible doesn't say heaven is reserved only for a few, and most believers will be second-class citizens in God's Kingdom. Heaven is the hope of every believer, because God has promised us "an inheritance that can never perish, spoil or fade kept in heaven for you" (1 Peter 1:4).
The best way to help your friend is to pray for her, and to ask God to help you point her to what the Bible teaches about heaven. Don't argue with her, however, for arguing is seldom productive (especially when someone is convinced you are the one who is wrong!). Follow instead the Bible's injunction: "Let your conversation be always full of grace, seasoned with salt, so that you may know how to answer everyone" (Colossians 4:6).
Why is this issue even important? The reason is because your friend apparently doesn't understand what Jesus has done for her. When He died on the cross, all our sins were placed on Him, and He took the punishment we deserve. Now He offers us salvation as a free gift free, because He fully paid the price for it.
We can't earn our salvation by our good works (as your friend believes); all we can do is receive it by turning to Christ and putting our faith and trust in Him. Make sure of your own commitment to Christ, then ask God to help you point your friend to the free gift He offers her of salvation in Christ.