If they are an LLC getting taxed as an S-Corp and they filed an 1120S, you have to amend the 1120S to get an amended K-1 before you can amend their personal returns. You can not file a Schedule C for that business if they are an S-Corp.

You can technically (not good accounting) complete an S-Corp return without a balance sheet if they meet the parameters for it - i.e. they have less than $250,000 in assets AND net receipts. That being said - I NEVER file a business return without a balance sheet. Just because the IRS doesn't require it for a small business doesn't mean that it doesn't exist. Every business has a balance sheet if their accounting is being done properly.