I agree Umarian, the harrower class would not add to the bonuses of the familiar of the previous class. However, the prerequisites for Improved Familiar include "Ability to acquire a new familiar, compatible alignment, and sufficiently high level."
If a character stops advancing in a class that grants an arcane familiar, they still have the ability to acquire a familiar based upon the wizard levels already taken. Page 82 of the core rule books states "if a familiar is dismissed, lost, or dies, it can be replaced 1 week later through a specialized ritual that costs 200 gp per wizard level." This ability is not contingent upon continued advancement as a wizard. For example, if a hero stopped advancing as a wizard, and started advancing as a fighter, they could still use the ritual ability to obtain a new familiar after advancing as a fighter. The ability to acquire a familiar is not contingent on class level. Thus, if an arcane caster advances their arcane spellcaster level with a non-familiar prestige class, they do not lose the ability obtain a new familiar if the prior familiar is dismissed, lost, or dies. With this in mind, it seems like a character with wizard level 5 and harrower level 2 could obtain improved familiars with an arcane caster level of 7. The three prerequisites would be met:
1. Ability to Acquire a new familiar: the hero has this ability from her 5 levels of wizard.
2. Compatible Alignment: I assume the chosen familiar would meet this requirement.
3. sufficiently high level: As I discussed in my last post, the arcane spellcaster level would total seven, even though the wizard class level is 5.
With all three prerequisites of the Improved Familiar feat satisfied, it seems as though a wizard 5 harrower 2 hero would be able to select from the improved familiars available to arcane spellcaster's of 7th level.