Personally it would be a cold day in hell when I disable avast to install an application. Basically I would like to know what they have to hide/fear from having your AV enabled during install; a time when I feel you need it most. So these occasions are rare and more the exception than the rule, such as when running other security software analysis, etc.
What is the application ?
Do you know why they say you have to disable ?
It is possible to disable avast, by right clicking on the avast tray icon, avast! shields control, Disable for X duration, see image, click to expand.