If I purchased materials from my vendor for a friend who owns his own company, and he has reimbursed me for the materials by check. How do I record that in QuickBooks?

I'd be glad to help you record this transaction in QuickBooks Online (QBO), @info-fibid-com.

You can add your friend as a customer in QuickBooks and create a billable expense. That way, you can bill the expense to him. 

First, let's make sure that this feature is turned on in the Account and settings. Here's how:

1. Go to Settings ⚙, then select Account and settings.
2. Click the Expenses tab.
3. From the Bills and expenses section, select Edit ✎.
4. Turn on the following:
   • Show Items table on expense and purchase forms
   • Track expenses and items by customer.
   • Make expenses and items billable.
5. Select Save, then click Done.

The next step is to bill your friend for that expense. 

1. Click + New and select the transaction (Bill, Expense, or Check) you want to create.
2. Choose the payee.
3. In the Category column, select the expense account for transaction.
4. Enter the description and amount of the expense, then select the Billable checkbox.
5. In the Customer column, choose the customer you want to bill for this expense.
6. Select Save and close.

To reimburse the cost, link the billable expense to your customer's invoice.

1. Click the + New icon and select Invoice.
2. In the Customer ▼ drop-down menu, select the customer you created a billable expense for. This opens the Add to invoice window.
3. Select Add on the billable expense you want to charge to your customer.
4. Click Save and close.

For complete steps, you can check out this article: Enter billable expenses.

Moreover, you can visit this guide to know about recording expenses in QBO: Enter and manage expenses in QuickBooks Online.

If you need help recording reimbursements or other entries that you find a little bit challenging, feel free to visit us back in the Community @info-fibid-com.

@info-fibid-com 

I would suggest not using billable expenses since these supplies were not for your business.  You do not want income or expenses showing on your P&L.  I would suggest keeping it simple: create an Other Current Asset account called 'Due from XYZ Corp' (your friend's business).  Then, when you enter the bill from your vendor, assign the newly-created 'Due from XYZ Corp' to the bill under Category Details.  Then, when you deposit the reimbursement from your friend's business, assign the same 'Due from XYZ Corp' (under Account) to the deposit.  Then, pay the vendor bill like normal.