how can such a workflow be created in sharepoint designer.
should start when a new item is created in listA
Condition > if any item in the listB exist with customer name as andrews then
Action > update that item in listB with field colA = orignal value in the field + 1
Else
Action > Create item in list B with field colA = 1 and other fields whatever not important.
Apparently you are making a counter grouped by a common denominator (a username in this case). Thats a common scenario but hard to go in step by step, luckily Microsoft made an example of this in their Service Pack.