Hi Mink,
So, you want to choose someone and see previous Absence Request history including a subtotal of hours taken?
If all of the data for all of the people is loaded in your form, you could do this with simple filtering (no code required).
However, this would be bad because a) the data is static, it doesn't refresh if others are filling out the form from a separate InfoPath instance, and b) the amount of data will be large and inefficient to download and filter all in one form.
My recommendation would be to promote the fields for Person's name, Date From and Total Hours to your SharePoint form library and then use a filter on this. If you don't have many people, you can filter it in the form, but as I said in (b) above, it's not ideal for large sets of data. If you have >100 absence requests/year I recommend using qRules to pass a field parameter to FilterOwssvr function on SharePoint. SharePoint will filter the list for you on the server and send you back just the items that match. Then, it will be faster.
Another option is to map your absence requests to a database. We've done a handful of Absence Requests in the past couple years and we tend to use SharePoint list or database to store the data.
Good luck!