Shreejana, nice question , its always good to determine proline content in fresh leaves, since proline has been proposed to act as an important compatible osmolyte and osmoprotective compound, acting as molecularchaperone in osmotic adjustment and protection of cellularstructures, proteins, and membranes during osmotic stress. 

Here is an excellent PDF describing three most widely used methods viz., using Isatin paper assay , colorimetric assay and HPLC assay ..out of which colorimetric is most cost effective ..

According to Bates 1973 fresh samples of  leaves were used to estimate proline content  in stressed plants but not dry samples.

yes, you could use  dried leaves samples. this is the method of Troll and Lindsley(1955). the method is as follows:Extract 200mg of ground leaf sample with 5ml of 95 %ethanol. do centrifuge and take the upper phase.dry the extract until dry ness. add 2ml of distilled water and  centrifuge. take 1 ml of the  upper phase and read absorbance at 490nm. make standard curve of proline andcalculate proline concentration in micrograme pergm  dry weight.

Here is a  protocol by Carillo et al  that  describes how to assay the amino acid proline with a ninhydrin-based simple, fast and harmless method using a microplate reader or a cuvette spectrophotometer. You can apply to both fresh as well as dried leaf samples..

If you are interested in free proline you can extract from dried material, but it's probably better to freeze-dry. I use kits from a company called Phenomenex (EZ:Faast) which can be used to detect all free amino acids within samples on either LC-MS or GC-MS systems. The kits are quite expensive but give excellent and reproducible results.